Is it possible to upgrade my Graphics Card to a newer one?

I wonder if I can replace my ati radeon 2600 pro for a newer PCI-e model of Graphics Card. If so, which one would you recommend?

My two main concerns are the available space inside my iMac (i.e. the dimensions of the new GPU card) and if I could attach the thermal device to one of those cards (may be dissassembling them from the plastic box where they are enclosed).

I even wonder if it would be possible to replace it for a newer nVidia card, but I'd prefer to stay with an ATI model.

Thank you for the answer. I have a 20" 2007 iMac and I have upgraded most of the known possible upgrades. I have upgraded the cpu to a 2.8 ghz 7900XT, 6gb ram and replaced the original HDD to a Samsung Evo 850 Pro 256GB. I really enjoyed the process and now I have Yosemite running on this machine. The only thing, I would like to do to this awesome iMac is to replace the Radeon 2400 XT with a better one.

I know you can upgrade it to a 2600 XT Radeon which would be a substantial upgrade but is there a possibility to upgrade to an 8800 GS Nvidia that is used in the 24"?

I have a A1224 imac 20". The graphic card installed is an ATI RADEON 2600 Pro ; this card is dying.

I went on yhe ifixit link indicated ; I see all steps to replace this card but it doesn't answer to the following question : is it possible to replace the original model by another one and if yes can you give me references of possible cards ?